Geoffrey of Monmouth wrote several works of interest. The earliest one to appear was Prophetiae Merlini or "The Prophecies of Merlin," which he wrote at some point before 1135. Geoffrey presented a series of apocalyptic narratives as the work of the earlier Merlin &ndash who, until Geoffrey's book came out, was known as "Myrddin". (It is assumed that Geoffrey changed the name of the seer to avoid an unwanted association with the French word merde.) The first work in a non-Welsh language about this legendary prophet, this was widely read &ndash and believed much as the prophecies of Nostradamus are centuries later.
Next was Historia Regum Britanniae – "The History of the Kings of Britain" – the work best known to modern readers. It claims to relate the history of the pre-Saxon kings of Britain, but much of it &ndash for example, the idea that Aeneas was the ancestor of the first line of British kings – is legend. It is one of the first texts to mention King Arthur and the first surviving text to mention King Lear (as King Leir), and it continued to influence British writers and historians for several centuries.
Lastly, Geoffrey wrote the Vita Merlini &ndash "The Life of Merlin" – at some point between 1149 and 1151. This is Geoffrey's own retelling of earlier Merddin legend from Welsh tradition.
All of these books were written in Latin, as were all learned works of the medieval period.
English Translations Available on the Web: